1. Field of the Invention
The present invention relates to a transflective liquid-crystal display device capable of both of reflecting display using extraneous light reflection and transmissive display using a backlight and an electronic device including the same.
2. Description of the Related Art
In the field of display devices, active-matrix display devices are now widely used which provide high display quality. The display devices have a switching element for each of a large number of pixel electrodes, easily providing the characteristics of large scale and high definition by reliable switching for each pixel electrode.
Recently, the demands for reducing power consumption and increasing the luminance of display with a largest possible pixel region are increasing. Accordingly, devices are in practical use which have a thick insulator film on the entire surface of an active-matrix substrate, on which reflective pixel electrodes are formed. Such a structure in which pixel electrodes are placed on the insulator film can adopt a structure in which no electrical shortage occurs between scanning lines and signal lines disposed on a layer under the insulator film and pixel electrodes disposed on a layer on the insulator film, which enables the pixel electrodes to be formed in a wide area so as to overlap on the lines. This allows almost all of the region having the switching elements such as thin film transistors (hereinafter, abbreviated as TFTs), the scanning lines, and the signal lines to be used as a pixel region that contributes to display, increasing the open area ratio to provide light display.
Since the liquid-crystal display mode using the reflective pixel electrodes cannot be used alone in a dark place, transflective liquid-crystal display devices are also widely used which include a backlight to allow the reflective liquid-crystal display device to be capable of partial transmissive display (refer to Japanese Unexamined Patent Application Publication Nos. 11-101992 and 2000-171794).
FIG. 17 shows an example of this conventional type of transflective liquid-crystal display device (refer to Japanese Unexamined Patent Application Publication No. 11-101992). In this liquid-crystal display device, a plurality of thin-film transistors 101 is disposed in line in the display region on a transparent substrate 100. The thin-film transistors 101 are coated with a transparent insulator film 102. The insulator film 102 has pixel electrodes 103 made of a transparent conductive film in pixel forming positions thereon. Light-reflective metal electrodes 106 are provided at positions except part of the pixel electrodes 103 such that they connect to the pixel electrodes 103. The transparent conductive film extends to contact holes 107 of the insulator film 102 to connect source electrodes 104 of the thin-film transistors 101 to the pixel electrodes 103 made of the transparent conductive film. The side opposite to the substrate 100 has a substrate 108, wherein between the substrates 100 and 108, a liquid crystal layer 109 is sandwiched. In FIG. 17, the description of a backlight disposed on the back of the substrate 100 is omitted.
The liquid-crystal display device with the structure of FIG. 17 is constructed such that the thin-film transistors 101 selectively apply power to the light-reflective pixel electrodes 106 to control the orientation of the liquid crystal, thereby controlling the transmittance of the liquid crystal to perform reflecting display. While the thin-film transistors 101 apply voltage also to the transparent pixel electrodes 103, the region of the pixel electrodes 103 serves as transmissive display region by transmitting the light from the backlight. Thus, a transflective liquid-crystal display device is achieved which is capable of both of the reflecting display using the light-reflective pixel electrodes 106 and the transmissive display using a backlight.
FIG. 18 shows another example of the conventional pixel structure applied to the transflective liquid-crystal display device (refer to Japanese Unexamined Patent Application Publication No. 2000-171794) In the pixel structure of this example, a large number of thin-film transistors 111 is disposed in line in the display region on a transparent substrate 110. The thin-film transistors 111 are coated with an insulator film 112. The insulator film 112 has light-reflective pixel electrodes 113 made of an aluminum electrode film in pixel forming positions thereon. Recesses 116 are provided in the insulator film 112 under the pixel electrodes 113. A gate insulator film 118 and a drain electrode 119 extend on the bottom of the recess 116 in order from the bottom. The region where the drain electrode 119 is formed is used as transmissive display region. In FIG. 18, the description of a substrate opposite to the substrate 110 and a liquid crystal sandwiched between the substrates are omitted.
The liquid-crystal display device with the structure of FIG. 18 is constructed such that the thin-film transistors 111 apply power to the light-reflective pixel electrodes 113 to control the orientation of the liquid crystal, thereby controlling the transmittance of the liquid crystal to perform reflecting display. While the thin-film transistors 111 apply voltage also to the transparent drain electrode 119 in addition to the pixel electrodes 113, the region of the transparent drain electrode 119 serves as transmissive display region by transmitting the light from the backlight. Thus, a transflective liquid-crystal display device is achieved which is capable of both of the reflecting display using the light-reflective pixel electrodes 113 and the transmissive display using the backlight and the transparent electrodes 119.
With the related-art liquid-crystal display device shown in FIG. 17, an electric field is applied to the liquid crystal layer 109 between the transparent substrate 100 and the opposed substrate 108 from the light-reflective pixel electrodes 106 and the transparent electrodes 103 to control the orientation of the liquid crystal to perform display.
For the duration of time the incident light to the liquid-crystal display device reaches a viewer as reflected light, the light passes through the liquid crystal layer twice in the reflecting display mode, while in the transmissive display mode, it passes through the liquid crystal layer only one time, thus posing the problems of giving unnecessary color and different color tone to the display. To solve the above problems, the above structure of FIG. 17 prevents a decrease in display quality caused by optical-path difference between the display modes by setting the thickness of the light-reflective pixel electrodes 106 larger than that of the transparent pixel electrodes 103, setting the thickness d1 of the liquid crystal layer thin in the reflecting display region, while setting the thickness d2 of the liquid crystal layer thick in the transmissive display region.
The structure of FIG. 17, however, has the problem that after the thick light-reflective pixel-electrode layer has been formed on the entire surface of the thin transparent electrodes 103, the pixel electrodes 106 must be formed only at necessary portions by patterning in a photolithography process, resulting in complicated process.
If even a little pixel electrode 106 is left on the transparent electrodes 103, it affects the transmittance of the liquid crystal. Thus, the thick pixel-electrode layer formed once on the transparent electrodes 103 must be removed completely. However, when the formed pixel-electrode layer is patterned by the photolithography process, the ground transparent electrodes 103 may be damaged.
With The liquid-crystal display device of FIG. 18, the thickness of the liquid crystal layer in the transmissive display mode is increased using the depth of the recess 116 formed in the insulator film 112 so that the thickness of the liquid crystal layer when light passes through the liquid crystal twice in the reflecting display mode and the thickness of the liquid crystal layer when light passes through the liquid crystal layer in the transmissive display mode are equalized.
However, in order to equalize the thicknesses of the liquid crystal in the transmissive display mode and the reflecting display mode, the depth of the recess 116 must be equal to that of the liquid crystal layer (cell gap), or significantly deep. Accordingly, when the thin light-reflective pixel electrodes 113 are formed along the inner wall of the recess 116, as shown in FIG. 18, there is a high possibility of a break in the pixel electrode layer around the periphery of the opening of the recess 116 from the problem of step coverage during film formation.
Increasing the light-reflective pixel electrodes 113 in thickness so as not to have a break can prevent the problem of the break; however, the bottom of the recess 116 will be filled with the thick pixel electrodes 113, resulting in the need for a deeper recess, thus posing the problem of difficulty in ensuring the liquid crystal layer with a desired thickness for the transmissive display mode.